This invention relates to drive circuits, and is particularly concerned with drive circuits using bipolar transistors in current mirror arrangements for operation at low supply voltages.
It is increasingly desirable to reduce the power dissipation and increase the operating bandwidth of electronic circuits. For these and other reasons, it may be desirable to use bipolar transistor technology to provide a drive circuit which operates at a low supply voltage, for example of the order of 1.5 volts, without transistor saturation which could decrease the operating bandwidth and increase current consumption and hence power dissipation of the circuit. In view of typical operating voltages of bipolar transistors, avoiding saturation presents a considerable problem with such low supply voltages, especially considering other factors such as variations of the supply voltage, variations of transistor voltages (e.g. base-emitter voltage) with temperature, and possible cascading of circuits.
An object of this invention is to provide an improved bipolar transistor drive circuit.
According to one aspect of this invention there is provided a bipolar transistor drive circuit for operation from a low voltage supply, comprising: a first current mirror circuit having an input and an output each coupled via a respective resistor to a low voltage supply and constituting respectively a signal input and a signal output of the drive circuit; and a circuit for determining an output voltage swing at the signal output, said circuit comprising a voltage reference, a compensation current mirror circuit having an input and having an output coupled to the signal input of the drive circuit, and a circuit for supplying to the input of the compensation current mirror circuit a current dependent upon the voltage reference and the supply voltage so that the signal output voltage swing is determined substantially independently of the supply voltage.
The circuit for supplying a current to the input of the compensation current mirror circuit can comprise a transistor having a base supplied with the voltage reference, a collector coupled to the input of the compensation current mirror circuit and via a resistor to the low voltage supply, and an emitter coupled via a resistor to a reference potential, for example circuit ground or 0 volts.
The drive circuit can comprise at least one further current mirror circuit connected in cascade with the first current mirror circuit, each further current mirror circuit having a signal input coupled to the signal output of the preceding current mirror circuit and an output coupled via a resistor to the low voltage supply and constituting a signal output of the respective current mirror circuit, and a respective compensation current mirror circuit having an output coupled to the signal input of the further current mirror circuit. In this case conveniently the voltage reference and the circuit for supplying a current to the input of the compensation current mirror circuit can be provided commonly for a plurality of the compensation current mirror circuits.
For handling differential signals as is often required for a drive circuit, each of the first and any further current mirror circuits can comprise a differential current mirror circuit having differential signal inputs and outputs each coupled via a respective resistor to the low voltage supply, and each compensation current mirror circuit can have two outputs coupled to the differential signal inputs of a respective one of the first and any further current mirror circuits.
The voltage reference can comprise a bandgap voltage reference source and a voltage scaling circuit for reducing a voltage provided by the bandgap voltage reference source.
The invention also provides a drive circuit comprising: a bipolar transistor differential current mirror having two differential current inputs and two differential current outputs all coupled via respective resistors to a low voltage supply, the differential inputs and outputs constituting differential signal inputs and outputs respectively of the drive circuit; a circuit comprising a bipolar transistor having a base supplied with a reference voltage, a collector coupled via a resistor to the low voltage supply, and an emitter coupled via a resistor to a reference potential; and a bipolar transistor current mirror having a current input coupled to the collector of said bipolar transistor and two current mirror outputs coupled to the differential signal inputs, said circuit and bipolar transistor current mirror serving to stabilize a signal voltage swing at the differential signal outputs.
The invention further provides a drive circuit comprising: a bipolar transistor current mirror having a current input and a current output each coupled via a respective resistor to a low voltage supply, the input and output constituting a signal input and output respectively of the drive circuit; a circuit comprising a bipolar transistor having a base supplied with a reference voltage, a collector coupled via a resistor to the low voltage supply, and an emitter coupled via a resistor to a reference potential; and a bipolar transistor current mirror having a current input coupled to the collector of said bipolar transistor and a current mirror output coupled to the signal input, said circuit and bipolar transistor current mirror serving to stabilize a signal voltage swing at the signal output.
In addition, the invention provides a drive circuit comprising: a first current mirror circuit comprising first and second bipolar transistors having emitters coupled to a reference potential, bases coupled together and to the collector of the first transistor, and collectors of the first and second transistors constituting a signal input and a signal output respectively and being coupled via respective resistors to a low voltage supply; a source of a reference voltage; a third bipolar transistor having a base supplied with the reference voltage, a collector coupled via a resistor to the low voltage supply, and an emitter coupled via a resistor to a reference potential; and a compensation current mirror circuit comprising fourth and fifth bipolar transistors having emitters coupled to a reference potential, bases coupled together and to the collector of the fourth transistor, and collectors of the fourth and fifth transistors coupled to the collectors of the third and first transistors respectively.
Another aspect of the invention provides a method of determining and stabilizing a signal voltage swing at an output of a current mirror circuit, comprising the step of reducing current at a signal input of the current mirror circuit, in dependence upon a reference voltage and a supply voltage of the current mirror circuit, using a compensation current mirror circuit, so that the output signal voltage swing is not dependent upon the supply voltage.
In this method the current mirror circuit and the compensation current mirror circuit can comprise bipolar transistors, and the supply voltage can be of the order of 1.5 volts. The method can be applied to each of a plurality of current mirror circuits connected in cascade. In addition, the current mirror circuits can comprise differential current mirror circuits having differential signal inputs and outputs.